GB2232056A

GB2232056A - Method for sealing container; thermal treatment of foodstuffs

Info

Publication number: GB2232056A
Application number: GB9000035A
Authority: GB
Inventors: Alan John Perryment
Original assignee: APV Baker Ltd
Current assignee: SPX Flow Technology Crawley Ltd
Priority date: 1989-04-22
Filing date: 1990-01-02
Publication date: 1990-12-05
Also published as: GB8909213D0; GB9000035D0

Abstract

Thermal sterilisation of a foodstuff contained in a rupturable container, said container comprising a basal tray member and an upper cover member secured to the tray member, comprises passing the container through a thermal treatment zone, and is characterised in that the container fed to the treatment zone is only partially sealed so as to permit venting of any expansion or contraction of the contents of the container during the thermal treatment, and in that the container is sealed whilst within or immediately adjacent the thermal treatment zone once the contents of the container have been reached the desired temperature. A microwave tunnel incorporates a mechanism for sealing a container within it. <IMAGE>

Description

TITLE: METHOD FOR SEALING CONTAINERS The present invention relates to a method for sealing containers, notably to a method for sealing containers containing a foodstuff which has been pasteurised or heat sterilised, so as to preserve the integrity of that container.

BACKGROUND TO THE INVENTION: Many foodstuffs are presented in a cooled or chilled state for sale to the public. In order to preserve the foodstuff, this is packed in a sealed container. However, with the current awareness of potential contamination or bacterial degradation of the foodstuff it is required that the foodstuff be sterilised or pasteurised by heat or other treatment before the goods are offered for sale. In one method for achieving this, the foodstuff is passed through a microwave heating tunnel or chamber where the foodstuff is heated to the desired temperature and retained at that temperature for a required period.

Whilst this treatment may reduce the problems of contamination with salmonella and other bacteria, physical problems arise in that the foodstuff must be presented in a container for transport, storage and sale which protects the foodstuff against fresh contamination after the heat treatment. Typically, such a container is provided as a rigid or semi-rigid plastics tray in which the foodstuff lies, and a transparent semi-rigid cover which is heat sealed around its periphery to the tray. If the foodstuff is packed in the container after the heat treatment, there is a risk that the foodstuff may pick up new contamination during the packing operation.On the other hand, if the foodstuff is heat treated when sealed into its container, problems arise in that the heat treatment will cause the release of steam from the foodstuff and expansion of the airspace around the foodstuff within the container. This may cause the container to burst, since it is of only light weight construction, destroying the integrity of the container.

In order to reduce this problem, it has been proposed to fabricate the container from heavier gauge material so that it will resist the expansion during the heat treatment.

However, this imposes cost penalties and requires the use of different manufacturing techniques, which are unacceptable for operators who already have existing foodstuff packing equipment.

We have now devised a simple and effective method by which the problem of expansion can be reduced and the integrity of the container maintained after the heat treatment.

SUMMARY OF THE INVENTION The present invention provides a method for the thermal treatment of a foodstuff contained in a rupturable container, said container comprising a basal tray member and an upper cover member secured to the tray member, notably by heat sealing the upper cover member to the tray member, which method comprises passing the container through a thermal treatment zone, characterised in that the container fed to the treatment zone is only partially secured to the tray member so as to permit expansion or contraction of the contents of the container during the thermal treatment, and in that the container is sealed whilst within or immediately adjacent the thermal treatment zone once the contents of the container have been reached the desired temperature.

As indicated above, it has hitherto been considered essential that the container of foodstuff should be totally sealed as soon as practical after the contents have been placed in the container so as to avoid the risk of contamination of the foodstuff. The method of the invention runs contrary to this established criterion by only partially sealing the container prior to the thermal treatment and completing the sealing during or immediately after the thermal treatment.

The invention can be applied to a wide range of foodstuffs and containers therefor, notably to prepared foodstuffs and meals. For convenience, the invention will be described hereinafter in terms of the heat treatment of a meal portion, for example a chicken portion or a portion of chille con carne, held in a generally rectangular plan shape container. The container of the invention is rupturable, this is it would break when subjected to the stress of the expansion or contraction of the contents and the air space above then when subjected to the thermal treatment. Such containers are typically made from a sheet plastic or other comparatively fragile material and metal can or other subtantial and rigid containers are not included within the term container as used herein.

Furthermore, the contents of the container do not fill the container as is required in a tin can or similar filoing and processing line, but there is an appreciable air space, typically 10 to 50% of the volume of the container, over the contents of the containers for use in the method of the invention.

The thermal treatment can be carried out using a wide range of methods, for example by steam autoclaving, by infrared or radiant heating. However, the invention can be applied to freezing methods where the bacteria are killed by rapid freezing of the foodstuff within the container, since this can cause contraction of the air space above the contents, leading to collapse of the container cover. The invention is of especial application in the heat teatment of the container using microwave energy in a tunnel or other chamber through which the container is transported on a rubber or other conveyor mechanism. For convenience, the invention will be described hereinafter in terms of the use of such a heat treatment method. The thermal treatment is carried out using conventional techniques and equipment, except as stated below.

The container comprises a conventional basal tray and transparent cover made from any suitable material.

Typically, the tray and cover will be made from a food grade polyvinyl or polystyrene resin as conventionally used in the food industry. The meal portion is placed in the tray and the cover then affixed using conventional techniques, for example by adhesive or crimping the cover and tray together along their contacting peripheries.

Since these are well known in the art, they will not be described in any detail herein. However, the invention is of especial application where the cover is secured to the tray member by means of friction or heat welding of the two components. For convenience, the invention will be described hereinafter in terms of a cover which is heat sealed to the basal tray member.

However, as stated above, the cover is not fully sealed to the tray. For example, the bar sealer used to seal one of the edge junctions between the tray and the cover does not extend for the full length of that junction or has a break along its length, so that there are one or more discontinuities in the heat seal. Typically, the discontinuity in the seal between the tray and the cover will be from 0.5 to 5 mms long to provide an adequate vent through which expansion can occur during the heat treatment stage. If desired, there may be more than one vent to a container, as when the cover is attached to the tray by a series of tack welds, but it will usually be preferred that there is only one such vent.

The vent may be merely an unsealed butt join between the cover and the tray. However, the cover frequently carries a peripheral flange or shoulder which locates upon the upstanding rim of the tray or vice versa. In such a case, the unsealed portion of the rim of the cover can be formed from a flexible material which adopts a shape closely aligned to that of the rim of the tray it contacts. The overlap between the tray and cover material will thus act as a temporary loose seal which can be readily blown open as the contents of the container expand.

For convenience, the invention will be described hereinafter in terms of a container in which the cover is made from a semi-rigid thermoplastic material so that there is a positive aperture between the cover and tray at the unsealed vent.

The container is fed to and thermally treated in the microwave tunnel in the conventional manner, at the conventional temperature and for the conventional time required to pasteurise the meal portion within the container. During such heat treatment, some steam may be given off from the meal portion and the air in the head space above the meal portion within the container will expand. Such expansion will vent through the discontinuity in the seal between the cover and the tray so that minimal expansion stresses are applied to the comparatively fragile cover, which thus remains intact. However, once the food portion has reached the desired treatment temperature, the majority of any venting of the expansion will have taken place and the container can then be sealed to provide a sterile environment within the container.

The sealing can be achieved by any suitable method, having regard to the material from which the tray and cover are made and the vent gap to be sealed. Thus, it will usually be preferred to apply a conventional bar sealer to the unsealed area. However, other sealing methods, for example a hot air sealer or the application of a thermoplastic sealer to the vent gap can be used. For convenience, the invention will be described hereinafter in terms of the use of a conventional heat sealer bar technique.

The vent gap can be sealed whilst the container is still within the heating zone. However, this may impose problems due to the interaction of the microwave energy on any metal components of the heat sealing mechanism. It is therefore preferred that the heat sealing be carried out when the container is retained at the desired temperature for the dwell time required to destroy the bacteria in the meal portion. Such dwell time is usually achieved by carrying the container out of the immediate range of the microwave heater elements but retaining them within the thermally insulated tunnel so that comparatively little heat is lost from the container. The heat sealer can be located within this dwell area of the tunnel and can be operated in the conventional manner.

Once the container has been sealed and the meal portion has dwelt at the desired temperature for the desired time, the container is discharged from the microwave tunnel in the conventional manner. If desired, and adequate sterility provisions have been made, the heat sealing of the container can take place immediately after the container is discharged from the tunnel, but before the contents of the container have begun to contract to any significant extent.

The method of the invention reduces the problems of rupture of the conventional fully sealed containers during the thermal treatment and requires the minimum of change to the conventional operations and equipment. The invention can thus be applied to existing container filling and sealing lines and to existing thermal treatment operations by simple modification of the container sealers and by the provision of a container sealer mechanism to a conventional microwave tunnel or other conventional thermal treatment equipment.

The invention therefore also provides a microwave tunnel which incorporates a mechanism for sealing a container within the tunnel. Preferably, the sealing mechanism is a bar heat sealer located within the dwell section of the tunnel.

By sealing the container after the majority of the expansion has been vented, the sealed container will be under slight vacuum once the contents have cooled to ambient temperature. Subsequent inspection of the container will readily establish whether this vacuum has been retained or not. Containers produced by the method of the invention can thus be readily inspected at the point of sale by the purchaser to ensure the status of the container without the need for expensive bacterial testing equipment or specialist expertise.

Claims

CLAIMS:

1. A method for the thermal treatment of a foodstuff contained in a rupturable container, said container comprising a basal tray member and an upper cover member secured to the tray member, which method comprises passing the container through a thermal treatment zone, characterised in that the container fed to the treatment zone is only partially sealed so as to permit venting of any expansion or contraction of the contents of the container during the thermal treatment, and in that the container is sealed whilst within or immediately adjacent the thermal treatment zone once the contents of the container have been reached the desired temperature.

2. A method as claimed in claim 1 wherein the thermal treatment comprises heating the contents of the container to a temperature and for a time sufficient to pasteurise the contents of the container.

3. A method as claimed in claim 2 wherein the heating is carried out in a microwave tunnel.

4. A method as claimed in any one of the preceeding claims wherein the cover and tray members are made from a thermoplastic material and the cover is heat sealed to the tray member at the junction between the cover and tray members, the heat seal being interrupted by one or more discontinuities which provide vent apertures to permit expansion or contraction of the contents of the container.

5. A method as claimed in claim 4 wherein the the container is sealed by heat sealing the discontinuities by the application of a heat sealer located within the thermal treatment zone.

6. A method as claimed in claim 4 wherein the discontinuities are sealed by welding the cover to the tray by means of a heat sealer located within the dwell zone of a micro wave tunnel.

7. A method for thermally treating a food portion within a thermoplastic tray/cover assembly in which the cover member is heat sealed to the tray member and in which there are discontinuities in the heat seal so as to provide vent apertures by which expansion of the contents of the container during the thermal treatment can be vented to minimise the risk of rupture of the container; and in which the container is passed through a microwave tunnel to pasteurise the contents of the container; and in which the discontinuities in the sealing of the container are sealed by the application of a heat sealer thereto once the contents of the container have reached the desired temperature, the said heat sealer being located within the microwave tunnel.

8. A method as claimed in claim 1 substantially as hereinbefore described.

9. A microwave tunnel which incorporates a mechanism for sealing a container within the tunnel.

10. A microwave tunnel as claimed in claim 9 wherein the sealing mechanism is a bar heat sealer located within the dwell section of the tunnel.